most of the Achievements for this game are really hard.
Single and multi-player
Difficulty does NOT affect achievements
Steam achievements do work offline
4 missable achievements (plus 2 unknown)
Sign in with Steam or Xbox to track your progress, and:
Finish a level with one life.
You will eventually get it simply by playing the game.
Defeat Nestor in less than 3 minutes.
Defeating Nestor under 3 minutes should be pretty easy
Defeat Nestor without using the Megablast weapon.
I did not even know about MEgablast when fighting him, so it should be easy
Own 4 fully upgraded weapons.
Just Upgrade your Favorite weapons
Take no hits on any level.
This one is really tricky, I am not sure if Shield Counts, since you only get it if you Alt-F4 out of the game after the level. Took plenty of tries to get it, but in the end I am not sure what unlocked it
Absorb 1000 bullets with your shield.
This seems pretty glitched too, it did not count the bullets absorbed, but it did unlock at some point when I exited the game.
Be careful when playing with fire.
Not sure which weapon did it, but it did unlock after exiting the game.
Find the hidden levels.
This Achivment has been removed
Die exactly 42 times during the whole game.
If you alt-F4 when you have 42 Deaths, the Achivment should Unlock
Defeat the malfunctioning Space Machine.
I do not know what triggered this Achivment, since I did not manage to finish the Omega level.
Kill no enemies except bosses.
Just keep trying to dodge everything, if you die you can still use checkpoints, this one took alot of time, Crashing into enemies also kills them and fails the achivment.